Nanomaterials with size lower than 100 nm are the base building blocks of a new discipline that has emerged in the last few years—nanotechnology. In effect, presently there is a huge interest all over the world in the very small, from scientific fields to politics itself, and nanotechnology is even often present in the speeches of the main world leaders.
It is presented as having the potential to become a technological discontinuity with implications in citizens' life, such as occurred in the past with the discoveries of the steam engine, the train, the automobile, the computer, and biotechnology.
All this excitement stems from the fact that nanometric-sized particles or structures of a certain material present a set of electrical, optical, magnetic, and mechanical properties that are different from that same material of larger size, namely: high plasticity, superhardness, lower melting point, transparency, high surface area by mass unit with consequential improvement of catalytic activity, low thermal conductivity, increased magnetic effect, high semiconductor luminescence, changes of color, and even obeying the laws of quantum mechanics, leading some scientists to propose that one might stand on the verge of a new state of matter.
In order to take advantage of the opportunities created by this set of novel properties and translate them into practical applications, in addition to the most common binary structures (with two elements) such as Al2O3, Zno, TiO2, ZrO2, ceramic nanomaterials usually require:                Oxides formed by combinations of multiple elements in several ternary (ZnFe2O4) or higher (LaSrCuO) crystalline structures;        Non-oxides with covalent/metallic binding crystalline structures of the Nitride type: AlON, ALN, SiAlON, etc.;        Composites, they result from the combination of two or more materials (Alumina/Zirconia) in a single particle, maintaining the individual crystalline structures of each Alumina and Zirconia.        Solid solutions, in this case, unlike the previous one, one of the components is “dissolved” in the other so that only a single continuous crystalline structure (different from the initial ones) is detectable, MgO—NiO and Alumina/Chromia (with innumerous applications in the optical fibers field) are two examples of this type.        
Conceptually, ceramic composites result from the combination of two or more nanometric materials, and as such they present a combination of mechanical, thermal, electrical, magnetic, and optical properties that a single product could not exhibit.
A common example is the alumina/zirconia composite. Alumina is a material with a vast range of applications due to its high elastic modulus, high wear and etching resistance, and stability at high temperatures. It has, however, some weak points, namely low fracture toughness and flexural strength. When combined with zirconia to form a nanocomposite, it acquires a toughness level that makes it suitable for applications in biomaterials, optical devices, and extremely demanding operational conditions.
Likewise, a zirconia electrode used in hydrogen production from water decomposition requires a combination of ionic and electronic conduction, being then necessary to prepare a nanocomposite together with Ceria and Yttria, ZrO2—CeO2—Y2O3.
In turn, one of the multiple technological challenges in the area of combining several elements in different crystalline structures is designing and obtaining the zinc aluminate (ZnAl2O4) nanometric spinel, which possesses properties of high tear strength at high temperatures, with applications in aerospace industry.
A group of different nanometric crystalline structures, namely the ternary ones of the type AB2X4, A2BX4, ABX4, and ABX3, wherein A and B represent cations and X represents an anion, is particularly important for a vast set of special applications, from superconductivity (LiTiO3) to energy storage (LiMn2O4) to paramagnetic properties exhibited by the spinel type structure—arising from the presence of unpaired electrons in some electronic levels.
An example of a covalent/metallic binding non-oxide is nanometric ALN, exhibiting a high thermal conductivity which makes it an excellent alternative to alumina in advanced electronic circuits where an excess heat is produced due to the high concentration of circuits, rising the temperature and limiting the speed of applications. Nanometric AlN can dissipate this heat by an extremely fast conduction that maintains the temperature stable.
In the four types of examples listed above, in addition to their physical properties that are usually important for the characterization of nanomaterials in general, namely the primary particle size, size distribution, particle morphology, chemical purity level, surface area by volume unit, surface characteristics, and crystallite size, the oxides of ternary and higher structures, the covalent/metallic binding non-oxides, the composites, and the solid solutions further present, as opposed to the simple and binary structures, critical requirements of chemical and crystalline phase homogeneity at the individual level of each particle which are extremely difficult to obtain, because it is necessary to combine several different elements during the synthesis, in addition to the difficulty of attaining and maintaining the nanometric size of the primary particles.
These requirements are only ensured if a fast and complete reaction without concentration or temperature gradients during the synthesis of the nanoparticles is available, whether we are dealing with materials with multiple crystalline structures, composites, or solid solutions.
In addition to these two aspects, having reproducible and continuous methods, not only on a laboratory scale, allowing the production of large amounts of ceramic materials of all types of nanometric-sized structures previously mentioned is a further obstacle.
The existing methods for the production of composites, solid solutions, or different ternary (among which the spinel type) and higher nanometric-sized crystalline structures are similar. In fact, obtaining a composite or a crystalline structure depends only on the ratio of the elements, the temperature and pressure of the synthesis, and the phase equilibrium diagram thereof. Thus, for example, zirconia and titania in a 1:2 ratio between 700 and 1200° C. form a structure ZrTi2O6. But in a 5:7 ratio between 1100 and 1800° C., however, the compound with the ternary structure ZrTiO4 is formed.
The known and mentioned methods are divided in three categories: solid, liquid, and gas, according to the phase in which the synthesis reaction occurs.
1. Solid Phase Methods
In this method, the final compound/composite is usually prepared from a solid state reaction between the different elements, followed by an intensive milling process until a size of about 200 nm is obtained. The main limitations of this low cost method, besides the difficulty in attaining sizes lower than 0.2 microns, are related with the presence of impurities, a non-uniform distribution of particle size, and essentially a defective homogeneity level of composites and compounds, arising from the incomplete diffusion reactions between the reagents.
2—Liquid Phase Methods
There is a set of recent methods in the liquid phase which share the fact of starting with a solution in which the initial elements are stequiometrically dissolved at the molecular level. By means of different techniques, such as for example, coprecipitation, sprinkling, or sol-gel, a precipitate in the form of hydroxide is formed. All these methods have, as weak points, the different solubilities of the various hydroxides with the pH—implying that the ratio of cations in the final hydroxide does not match the ratio in the starting solution—, low production yields, and the requirement of subsequent steps, such as for example, calcination, for conversion to the oxide, and milling.
In combustion synthesis, a stequiometric amount of nitrates is dissolved in the minimum amount of water required and then an amount of fuel is added. This mixture undergoes heating at a temperature that can range between 200 and 500° C., and some minutes after the ignition a compound is obtained. The weakest point of the method, in addition to the operational discontinuity, is the high internal porosity of the particles, which is highly disadvantageous when conducting a subsequent sintering stage.
3. Gas Phase Methods
3.1. Low Pressure
It is presently being developed a set of alternative low pressure, gas phase methods, such as for example, the aerosol or pyrolysis synthesis, based in the production of a gas suspension that results in extremely fine particles by condensation. The main weak point of this route is the very low production capacity and the difficulty in obtaining complex (ternary) structures and composites.
In turn, the method proposed by the present invention, that is, detonation of a water/oil (W/O) emulsion, may be included in a new category (high pressure) of the gas phase reaction. The high pressure gas phase reaction has several advantages comparatively to the low pressure method, namely the fact that it allows combining and obtaining a large set of materials with different crystalline structures and nanometric composites in a single stage, in large amounts, and with high phase homogeneity, from the high pressure reaction of the various elements in the gas phase.
The production of nanomaterials by this method comprises four stages:    a) Preparation of the base (W/O) emulsion:
The formulation flexibility of the emulsion allows including in its composition a large set of precursors, from metals, metal alloys to different metal salts, which constitute the precursors that will transform into a all range of crystalline structures and composites;    b) Detonation reaction of the (W/O) emulsion with formation of a gaseous plasma:
The extremely fast detonation reaction rates (in the order of microseconds) generate high pressures which ensure a complete transformation degree of the precursors into a gas plasma that already contains the required materials/composites;    c) Condensation to form nanoparticles:
In order that the condensation phenomenon of the materials and composites occurs and results in the production of a large amount of nanometric-sized particles, the gaseous plasma has to achieve high pressures in a high supersaturation state. The size of the first condensates will decrease with increasing supersaturation degree, as measured from the relation between the reaction pressure and atmospheric pressure (P/PO). The pressure levels generated by the detonation of the emulsion ensure maximization of the (P/PO) relation and consequential production of nanometric-sized particles;    d) Control and preservation of the nanometric size of the particles:
After the formation of the first particles, the nanometric state will be rapidly destroyed by the beginning of the coagulation/coalescence phenomenon among particles, being therefore indispensable a fast cooling rate as soon as they are formed, or their dispersion in a high speed gas flow, for controlling the process.
The method of synthesis for nanometric-sized ceramic materials proposed by the present invention ensures a fast cooling rate and dispersion in a high speed gas flow through the ability to regulate the detonation temperature and the residence time of the particles in the higher temperature zones of the reactor.
In summary, the method proposed by the present invention combines a set of requirements which allow obtaining nanometric-sized ceramic materials with multiple crystalline structures, such as:                Oxides with binary, ternary, or higher crystalline structures        Non-oxides with crystalline structures of the nitride type        Composites        Solid solutionswhich, in addition to the high chemical and phase homogeneity level, exhibit a group of complementary properties adjustable according to the final applications, such as: uniform distribution of the primary particles between 15 and 100 nm, chemical purity level higher than 99.99%, surface areas by mass unit between 5 and 500 m2/g, and crystallite size below 50 nm, and real particle densities higher than 98% of the theoretical density.        
The use of the (W/O) emulsion concept for the production of nanoparticles is referred to in the document by Takoa Tami, Kazumasa Takatari, Naoysashi Watanabe, and Nabuo Kaniya, Metal Oxide powder synthesis by the Emulsion Combustion Method, Journal of Materials Research (1997). This document presents a new method for the synthesis of nanometric alumina powders “Emulsion Combustion Method” (ECM) from the combustion of a (W/O) emulsion with atmospheric air, affording hollow alumina particles. The method of the present invention, however, uses a different regime, i.e., it uses detonation instead of combustion, and does not make use of external air.
On the other hand, the use of the detonation process for explosives in the synthesis of materials with special properties started about thirty years ago, and since then has been focused on the production of nanodiamonds (a special structure of carbon), as an example the one referred to in the document by Fenglei H. Yi T., Shouurong Y., Synthesis mechanism and technology of ultrafine diamond from detonation, which discloses the use of carbon-rich military explosives for the production of nanodiamonds by means of detonation. The present invention is different from the subject-matter taught in this document in that it uses two (W/O) emulsions, one of them containing multiple precursors presenting a detonation regime at a temperature lower than 2000° C., to afford ceramic composites and a multiplicity of crystalline structures, whereas in this document use is made of military explosives distinct from the (W/O) emulsions, with a detonation temperature higher than 3500° C.
More recently, the detonation method is referred to in a set of other documents and publications, namely:
U.S. Pat. No. 5,855,827 describes a cyclic detonation process for the production of ceramic coatings on different substrates or production of micrometric and nanometric powders. The detonation takes place in a gas mixture containing a metal suspension of extremely thin granulometry. The process of the present invention distinguishes from the latter in that it uses two (W/O) emulsions in the liquid or solid phase, thereby allowing a better control of the conditions for the synthesis of nanomaterials and providing ceramic composites and various (ternary or higher) crystalline structures.
EP 1577265 discloses an industrial process for the production of fine (micrometric) alumina (Al2O3) powder from a cyclic detonation process of granulated aluminium mixed with an oxidizing agent. The present invention distinguishes from the latter in that it incorporates several types of metallic precursors, such as for example, nitrates, sulphates, acetates, in a (W/O) emulsion that exhibits a detonation regime at a temperature lower than 2000° C., providing nanometric instead of micrometric-sized particles that allow obtaining composites and multiple (binary, ternary, and of the nitride type) crystalline structures.
The document by Chiganova, G. A, Detonation synthesis of ultrafine alumina, Inorganic Materials, MAIK Nauka/Interperiodica ISSN 0020-1685 (Printed) 1608-3172 (Online) Vol. 41 No 5, May 2005, pp. 468-475, discloses the use of the explosion energy to accelerate and oxidize aluminium in very thin foils inside a chamber with oxygen, obtaining nanometric alumina. In the present invention, the detonation reaction of the different precursors, such as metals, alloys, nitrates, sulphates, or alike, takes place within the composition of one of the (W/O) emulsions, whereas in this document the aluminium reaction occurs later in a gas chamber, obtaining alumina with transition (non-stable) crystalline phases and some unreacted aluminium.
The document by R. Y Li, X. J. Li, and X. H. Xie, Explosive synthesis of ultrafine Al2O3 and effect temperature of explosion, Combustion, Explosion and Shock Waves, Vol. 42, No 5, pp. 607-610, 2006, teaches the production of nanometric Al2O3 in several metastable phases from the decomposition of Al(NO3)3 mixed in a military explosive, RDX, during detonation. The final product alumina exhibits several contaminations. The method presented in the present invention is different in that it uses two (W/O) emulsions, one of them containing multiple precursors that are an integral part of its composition (at the molecular level), which allow obtaining a group of non-binary, such as ternary, higher, and non-oxide structures with high purity level.
PT 103838, Nanocrystalline spherical ceramic oxides, process for the synthesis and uses thereof (“Óxidos cerâmicos esféricos nanocristalinos, processo para a síntese e respectivas utilizações”), discloses a process for the synthesis of polycrystalline, spherical, micrometric particles (less than 40 microns) composed by nanometric crystals for applications in ceramics industry. The process for the preparation and detonation of the (W/O) emulsion of the present invention distinguishes from the latter in that: a) it uses two W/O emulsions, one of them exhibiting detonation temperatures lower than 2000° C. that allow obtaining nanometric-sized particles, unlike the process of the cited document which affords only micrometric-sized particles because it is conducted at temperatures higher than 2000° C.; b) it uses simultaneously several metallic precursors that allow obtaining oxides with tertiary and higher structures, composites, and solid solutions, whereas the method disclosed in PT 103838 allows the synthesis only of oxides with binary structures (two elements) oxides because it uses a single precursor per composition; c) it uses fuels soluble in the aqueous solution (internal phase) allowing the formation of non-oxide compounds, such as nitrides, carbides, and hydrides, that were not obtainable by the process described in said document.
In this way, the process of the present invention enables the synthesis of nanometric-sized ceramic materials with different covalent/metallic binding crystalline structures, homogeneous distribution of the primary particles, high chemical purity level, crystallite size below 50 nm, and real particle densities higher than 98% of the theoretical density.
The document by X. J. Li, X. OUYANG, H. H YAN, G. L SUN, and F. MO, Detonation synthesis of TiO2 nanoparticles in gas phase, Advanced Materials Research Vol. 32 (2008) pp. 13-16 (online), discloses the synthesis of TiO2 nanopowders from the detonation of a gas mixture of hydrogen, oxygen, and titanium chloride. Unlike the present invention, however, this method is limited to room temperature-gaseous precursors, whereas the method of the present invention uses a liquid state (W/O) emulsion and allows the use of a multiplicity of liquid and solid precursors as part of that emulsion.